Heat exchangers, such as the steam generators of pressurized-water nuclear reactors, generally comprise a very thick tube plate, throughwhich pass tubes which are flush with one face of this tube plate forming its inlet face and which project on the other side of the tube plate, the outlet face of which is in contact with the interior of the steam generator receiving the feed water to be vaporized. The inlet face of the tube plate opens into the inner volume of a two-part water box, which ensures the distribution and recovery of the pressurized water circulating in the tubes and carrying the heating and vaporizing heat from the feed-water. Each of the tubes of the steam generator, curved round its upper part, has one end opening into one part of the water box and its other end opening into the other part of this water box.
The tubes of the steam generator, which form a bundle are crimped inside holes of suitable diameter which extend through the tube plate practically over the entire thickness of this plate. The crimping of the tubes in the tube plate involves rolling their wall against the wall of the holes in the plate, with a result that on the same side as the outlet face of the tube plate, in the wall of each of the tubes, there is a transition zone between the deformed part and the non-deformed part of the tube. The stresses in this transition zone of the tube are usually reduced by carrying out, after crimping, a slight diametral expansion of the tube in the transition zone. Nevertheless, a concentration of relatively high stresses still persists in this zone of the tube, which is consequently subjected to increased local corrosion, both from its inner surface in contact with the pressurized water and from its outer surface in contact with the feed water.
When maintenance is carried out on the steam generators of a shut down nuclear reactor, nuclear reactor, it is necessary to inspect the tubes, particularly in order to ascertain their corrosion resistance in the transition zone, after a certain period of operation. These inspections may require the extraction of a tube portion surrounding the transition zone.
Such extraction is useful only if it can be carried out without deformation or change in the surface condition of the tube, in the zone to be inspected.
There have been proposals for processes and devices which make it possible to extract a tube crimped in a tube plate either by wrenching it out mechanically or by contraction of the tube obtained by heating or by supercooling.
Where mechanical wrenching is concerned, the extractor produces lines and marks on the inner and outer surfaces of the tube, and as regards contraction by thermal means the tube experiences changes in the metallurgical phase and in the surface state. In both cases, the portion of the tube to be inspected has undergone a modification which makes the results of the appraisal doubtful.
French patent No. A-2,452,352 describes a device which makes it possible to extract the tubes by mechanical wrenching and which comprises a split bush and a knob to spread apart the various parts of the bush diametrically. This bush has teeth on its outer surface which dig into the inner part of the tube.
French patent No. A-2,220,326 describes an expansion device which is placed inside the tube and which comprises an assembly of elastic sleeves separated by rigid rings. An inner sliding core connected to the upper end sleeve causes the elastic sleeves to expand as result of axial displacement under the effect of a pull. This ensures that the device is attached, and a pull is subsequently exerted on it to wrench out the tube.
French patent No. A-2,491,249 describes a complex device which makes it possible to cut off and withdraw a steam-generator tube and divide it into sections. This device comprises a split bush provided with attachment teeth.
These devices of the prior art which ensure that the tube is wrenched out mechanically produce marks on the inner and outer surfaces of the tube and are generally of complex structure.